Transmission of data in radio communication systems such as mobile cellular communication systems have become widespread and in order to fully exploit the scarce radio resource, it has become increasingly important to control transmission such that data throughput is optimised
In a cellular communication system each of the remote terminals (such as mobile stations, subscriber units, user terminals etc) communicates with typically a fixed base station. Communication from the remote terminal to the base station is known as uplink and communication from the base station to the remote terminal is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of remote terminals 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a remote terminal moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the remote terminal and the base station of the first cell, to being between the remote terminal and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. The base station themselves may also be considered part of the network. A call from a remote terminal is routed through the fixed network to the destination specific for this call. If the call is between two remote terminals of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other remote terminal currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a remote terminal and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the remote terminals and the base stations. This spectrum must be shared between all remote terminals simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of remote terminals can be accommodated in the same wideband spectrum by allocating different codes for different remote terminals. Codes are chosen to minimise the interference caused between remote terminals typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in ‘Spread Spectrum CDMA Systems for Wireless Communications’, Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
Traditional traffic in mobile cellular communication systems has been circuit switched voice data where a permanent link is set up between the communicating parties. In the future, it is envisaged that data communication will increase substantially and typically the requirements for a remote terminal to transmit data will not be continuous but will be at irregular intervals. Consequently it is inefficient to have a continuous link set up between users and instead a significant increase in packet based data traffic is expected, where the transmitting remote terminal seeks to transmit the data in discrete data packets when necessary. An example of a packet based system is General Packet Radio Service (GPRS) introduced to the Global System for Mobile communication (GSM). Further details on data packet systems can be found in ‘Understanding data communications: from fundamentals to networking, 2nd ed.’, John Wiley publishers, author Gilbert Held, 1997, ISBN 0-471-96820-X.
In such wireless packet data systems, retransmission schemes are used to ensure that data packets are received without errors. One such method is an automatic request (ARQ) scheme, where the receiver determines if any errors are received in the data packets, and if so requests retransmission of the packets in error.
ARQ mechanisms are thus employed to ensure error free message transfer between two entities. The receiving-end transmits a feedback message, referred to as ACK/NACK (ACKnowledge/Not ACKnowledge) message, to the transmitting-end to indicate the correct or incorrect reception of the transmitted packets. The ARQ mechanism allows the system to operate at a higher packet error rate referred to as Block Error Rate (BLER).
In a multi-rate packet data system, two types of ARQ techniques are envisaged. The first one is based on allocating a user with a code that matches its radio condition at a specified performance figure. This is called pure Link Adaptation (LA). The second method is based on allocating the highest code rate to a user but improving its link performance by code combining the corrupted packets. This code combining is performed by decoding the received signal using data bits gathered over a plurality of data packets. This is called pure Incremental Redundancy (IR). In practice, combined LA/IR will be used due to hardware and higher layer protocol limitations. In this mode, while IR can take place, the user is allocated the best coding scheme on a regular basis. An example of a communication system employing these techniques is the Enhanced GPRS (EGPRS) which is being introduced in many GSM systems. Further description of EGPRS is available in “EDGE: enhanced data rates for GSM and TDMA/136 evolution ”by Furuskar, A.; Mazur, S.; Muller, F.; Olofsson, H. in IEEE Personal Communications; Volume: 63, June 1999.
In such wireless data packet systems it is essential that data packets or messages are communicated reliably but with the minimum of resource requirement. For example in a GSM system, it is desirable to communicate at high channel data rates and therefore low levels of coding but at the lowest possible power level to reduce interference.
In a multi-code packet data system such as GPRS, the code allocation algorithm is the central point of optimising the throughput and delay of a user and the system as a whole. Link adaptation is the process of selecting the best coding scheme that satisfies a target performance based on the prevailing channel conditions. However, inaccuracies in the channel estimate prevent the selection of the optimum code, which results in imperfect link adaptation.
Consequently these known systems are wasteful in terms of resource utilisation and an improved system for transmitting data is therefore desired.